The proton inventory technique, which involves measurements of reaction rates in mixtures of protium oxide and deuterium oxide, is being rigorously evaluated as a method for determining the transition state structure for several biologically important reaction types. The influence of reaction conditions such as temperature, ionic strength, pH, and the presence of added organic cosolvents is being outlined in a systematic way for a variety of reaction types which are important as models for biological processes. This work will allow an assessment of the proton inventory technique in assigning observed solvent isotope effects in intermolecular, intramolecular, and enzymatic catalysis to specific sources. The results will provide information about the importance of proton transfers in catalysis and the factors which influence these transfers. The technique has been applied to investigations of the water-catalyzed hydrolysis of 1-acetyl-1,2,4-triazole, 1-acetylimidazolium ion, p-nitrophenyl dichloroacetate, propionic anhydride, succinic anhydride, and N-acetylbenzotriazole. It appears, based on present results, that the proton inventory is fairly independent of reaction conditions. More studies are required, however.